1. Field of Invention
The present invention relates generally to video encoding, and more specifically to multi-pass variable bit rate encoding.
2. Background of Invention
Encoded digital video, such as video encoded according to the Moving Picture Expert's Group Version 2 (“MPEG2”) standard and stored on a Digital Video Disk (“DVD”), is very commercially popular today. Contemporary video encoders are expected to produce high quality results, and to offer a wide variety of user controls, such as multi-pass variable bit rate encoding. In multi-pass variable bit rate encoding, an encoder makes multiple passes through a video sequence in order to attempt to set optimized bit rates for the frames thereof.
Because the bit rate for video encoded on a DVD or similar medium can vary per frame, it is desirable to utilize variable bit rate encoding to maximize the output quality, as the number of bits needed to describe frames of a video sequence varies based on content and other factors. With fixed sized media, it is required to store an entire data image (e.g., a video sequence describing a film) in a fixed space (e.g., one side of a DVD). By varying the bit rate per frame such that individual frames are encoded at optimal bit rates, an attempt is made to maintain roughly constant quality throughout the video sequence.
For variable bit rate encoding, it is desirable for the encoder to make multiple passes through the video sequence. Because the bit rate of different frames will vary as a function of frame complexity, the encoder can build a frame complexity profile during the first-pass, and then encode the sequence according to the complexity profile during a second-pass.
Multi-pass variable bit rate encoding is known, but requires a trade off between encoding a video sequence at a target average bit rate (e.g., encoding a video sequence to fit on one side of a DVD) and the quality of the encoded video sequence. Multi-pass variable bit rate encoding as it exists in the prior art reduces the bit rate of complex frames by lowering the quality of those frames as needed to hit a target average rate. Unfortunately, this results in inconsistent quality across the video sequence as a whole, because more complex frames are encoded for lower quality than that of less complex frames.
What is needed are robust multi-pass variable bit encoding methods, systems and computer program products that allow encoding of a video sequence at a target average bit rate while still maintaining a substantially consistent quality across the video sequence as a whole.